Crude oil from which desired gaseous and liquid fuels are made contain a diverse mixture of hydrocarbons and other compounds which vary widely in molecular weight and therefore boil over a wide range. For example, crude oils are known in which 30 to 60% or more of the total volume of the oil is composed of compounds boiling at temperatures above about 650.degree. F. Among these are crudes in which from about 10% to about 30% or more of the total volume consists of compounds so heavy in molecular weight that they boil above about 1025.degree. F. or at least will not boil below 1025.degree. F. at atmospheric pressure.
Because these high boiling components of crude oil boiling above 343.degree. C. (650.degree. F.) are unsuitable for inclusion in gasoline or in some higher boiling liquid hdyrocarbon product fuels, the petroleum refining industry has developed processes for separating and/or breaking down the molecules of the high molecular weight, high boiling compounds into smaller molecules which do boil over a more appropriate boiling range. The cracking process which is most widely used for this purpose is known as fluid catalytic cracking (FCC). Although the FCC process has reached a highly advanced state, and many modified forms and variations have been developed, their unifying factor is that a restricted boiling range hydrocarbon feedstock is caused to be cracked in a riser reactor at an elevated temperature in contact with a cracking catalyst that is suspended in the feedback under cracking conditions providing a temperature in the range of about 510.degree. C. to about 593.degree. C. (950.degree. F. to about 1100.degree. F.) at the riser outlet. Upon attainment of a desired degree of molecular weight and boiling point reduction in the catalytic cracking operation the catalyst is separated from the hydrocarbon conversion fuel products.
Crude oils in the natural state contain a variety of materials which deposit troublesome deactivating materials on FCC catalysts. Only a portion of these troublesome materials can be economically removed from the crude oil or from the cracking catalyst. Among these troublesome materials are coke precursors (such as asphaltenes, polynuclear aromatics, etc.), heavy metals (such as nickel, vanadium, iron, copper, etc.), lighter metals (such as sodium, potassium, etc.), sulfur, nitrogen and others. The lighter metals can be substantially removed by desalting operations, which are part of the normal procedure for pretreating crude oil for fluid catalytic cracking. Other materials, such as coke precursors, asphaltenes and the like, tend to break down to form hydrocarbonaceous deposits or coke during the cracking operation, which deposit on the catalyst and thus impair further conversion contact between a hydrocarbon feedstock and the catalyst. The heavy metals in the residual oil feed transfer almost quantitatively from the feedstock to the catalyst surface.
If the catalyst is reused again and again for processing additional heavy residual oil feedstock, which is usually the case, the heavy metals in the feedstock comprising nickel, vanadium, iron and copper accumulate on the catalyst to the point that they unfavorably alter the composition of the catalyst and/or the nature of its catalytic effect upon the feedstock. For example, vanadium tends to form fluxes with certain components of commonly used FCC catalysts, lowering the melting point of portions of the catalyst particles sufficiently so that they begin to sinter, agglomerate and become ineffective cracking catalysts. Accumulations of vanadium and other heavy materials, especially nickel, also "poison" the catalyst. They tend in varying degrees to promote hydrogenation, dehydrogenation and aromatic condensation, resulting in excessive production of carbon and gases with consequent impairment of liquid fuel yield. An oil such as a crude or crude fraction or other heavy oil fraction that is particularly abundant in nickel and/or other metals exhibit similar behavior, also contains relatively large quantities of coke precursors. Such heavy oil feeds are referred to herein as carbo-metallic oil feeds and represent a particular challenge to the petroleum refiner to achieve economic conversion thereof to more useful fuel products comprising gasoline, light and heavy cycle oil products.
Several proposals in the prior art involve treating a heavy oil feed to remove the metal therefrom prior to cracking, such as by hydrotreating, solvent extraction and complexing with Friedel-Crafts catalysts, but these techniques mullify the refined costs and arecriticized as unjustified economically in the present crude oil available environment. A combination cracking process comprising separate "dirty oil" and Clean oil" processing units has been proposed. Still another proposal blends residual oil with gas oil and controls the quantity of residual oil in the mixture in relation to the equilibrium flash vaporization temperature at the bottom of the riser type cracking unit employed in the process. Still another proposal subjects the feed to a mild preliminary hydrocracking or hydrotreating operation before it is introduced into the cracking unit. It has also been suggested to contact a carbo-metallic oil such as reduced crude with hot toconite pellets to produce gasoline. This is a small sampling of the many proposals which have appeared in the patent literature and technical reports.
Notwithstanding the great effort which has been expanded and the fact that each of these proposals overcomes some of the difficulties involved, conventional gas oil FCC practice today bears mute testimony to the dearth of carbo-metallic oil-cracking techniques that are both economical and highly practical in terms of technical feasibility. Some crude oils are relatively free of the high boiling coke precursors or heavy metals or both. The troublesome components of crude oil are for the most part concentrated in the highest boiling fractions thereof. Accordingly, it has been possible heretofore to largely avoid the problems of Conradson carbon coke precursors and heavy metals accumulation by sacrificing some liquid fuel yield potentially available from the highest boiling vacuum bottom portions of crude oils. More particularly, the more conventional gas oil FCC practice has employed as a part of the gas oil feedstock that fraction of crude oil which boils at about 343.degree. C. (650.degree. F.) up to about 538.degree. C. (1000.degree. F.) but absent vacuum bottoms material. Such fractions are thus relatively free of heavy metal and Conradson carbon contamination. Such feedstock comprising atmospheric and "vacuum gas oil" (VGO) is generally prepared from crude oil by distilling off the atmospheric gas oil fraction or middle distillate boiling below about 600.degree. or 650.degree. F. at atmospheric pressure and then separating by vacuum distillation vacuum gas oils boiling from about 600.degree. or 650.degree. F. up to about 1000.degree. or 1025.degree. F. end boiling point.
A gas oil of atmospheric distillation boiling above 600.degree. F. (316.degree. C.) and/or vacuum distillation gas oils are used as the feedstock for conventional gas oil FCC processing. The heavier vacuum resid or vacuum bottoms product of crude oil distillation is normally employed in a variety of other purposes, such as for the production of asphalt, residual fuel oil, #6 fuel oil, or marine Bunker C fuel oil. This vacuum resid is now considered to represent a great waste of the potential value of this bottom portion of the crude oil, especially in light of the great effort and expense which the art has been willing to expend in the attempt to produce generally similar materials from coal and shale oils.
The present invention is concerned with the upgrading of residual oil comprising light and heavier gas oils and the heavier fractions of crude oils containing substantial quantities of coke precursors, heavy metals, and other troublesome residual components under operating conditions thereby increasing the overall yield of gasoline and other desired liquid fuels from a given crude oil. The method and apparatus of this invention is uniquely advantageous in several respects for upgrading high boiling carbo-metallic residual oils in a technically advanced manner to achieve thermal upgrading of poor quality feed more suitable for zeolite catalytic cracking thereof and the regeneration of fluid solid particles used in the combination operation for the purpose herein discussed.
In general, the coke-forming tendency of an oil can be ascertained by determining the weight percent of carbon remaining after a sample of that oil has been pyrolized. The industry accepts this carbon value as a measure of the extent to which a given oil tends to form non-catalytic coke when employed as feedstock in a catalytic cracker. Two established tests are recognized, the Conradson Carbon ASTM Method D189 and Ramsbottom Carbon tests, described in ASTM Test No. D524-76. In conventional gas oil FCC practice, Conradson Carbon values generally less than 2 and Ramsbottom Carbon values on the order of about 0.1 to about 1.0 are regarded as indicative of an acceptable gas oil feed. The present invention is concerned with the use of hydrocarbon feedstocks which have higher Conradson Carbon values in the range of 2 to 12 and higher Ramsbottom Carbon values above 1.0 and thus provide substantially greater potential for coke formation than is more usually obtained with gas oil FCC feeds of less than 2 Conradson Carbon value.
According to the more conventional prior art FCC practice, the heavy metal content of a feedstock for FCC processing is limited to a relatively low value, e.g., about 0.25 ppm Nickel Equivalents (nickel plus vanadium) or less. The present invention is concerned with the processing of feedstocks containing a concentration of metal contaminants substantially in excess of this value, and which therefore have a significantly greater potential for rapidly accumulating on and poisoning of the catalyst beyond economic recovery thereof.
In conventional FCC practice in which a circulating inventory of catalyst is used again and again in the processing of fresh gas oil feeds with periodic or continuing addition of fresh catalyst and withdrawal of equilibrium catalyst, the metal content of the catalyst is maintained at a level which may for example, be in the range of about 200 to about 600 ppm Nickel Equivalents. The process of the present invention is concerned with the use of catalyst having accumulated a substantial amount of metals, and which therefore have a much greater than normal tendency to promote undesired reaction of dehydrogenation, aromatic condensation, gas production or coke formation. A high metals accumulation of 1000 to 3000 ppm is normally regarded as quite undesirable in FCC processing.
The composition and molecular structure of hydrocarbon components comprising residual portions of crude oils and particularly that portion thereof referred to as vacuum resid affects substantially that level of hydrocarbonaceous material deposited on solids used to thermally and/or catalytically upgrade the residual oil feed to form liquid fuel products comprising gasoline. Thus, the higher the molecular weight, the higher the Conradson Carbon value of the feed and thus the higher will be the deposition of hydrocarbonaceous material on the fluid solid particles. The catalytic processing of feeds comprising components boiling above about 1025.degree. or 1050.degree. F. and comprising the vacuum resid portion of a hydrocarbon feed necessarily will increase the deposition of the carbonaceous material on the fluid solid particles during residual oil conversion. This deposition of carbonaceous material comprising hydrogen and more usually referred to simply as coke, deactivates the catalyst particles. The removal of deposited carbonaceous material from catalyst is accomplished by combustion in a separate regeneration system provided for the purpose.
It is well known that an increase in the level of coke deposition on catalyst particles will increase the combustion temperature encountered unless appropriate precautions are taken. This may be controlling by reducing, for example, the catalyst circulation rate by reducing the oxygen concentration of the combustion supporting gas, by using higher temperature particles, by providing indirect heat exchange means within the bed of solids in the combustion zone and combinations of the above. The patents of Medlin et al, Nos. 2,819,951; McKinney 3,990,992; and Vickers 4,219,442 disclose fluid catalytic cracking processes using dual combustion zone regenerators with cooling coils in particularly the second regeneration zone. The use of catalyst coolers which are external to the regeneration or coke combustion zone is also known in the prior art. Some of these patent disclosures include Harper Nos. 2,970,117; Owens 2,873,175; McKinney 2,862,798; Walson et al 2,596,748; Jahnig et al 2,515,156; and Berger 2,492,948.
The present invention on the other hand departs in several respects from the disclosure of the prior art by providing catalyst regeneration systems considered more suitable for the combustion removal of high levels of carbonaceous material deposits of residual oil conversion from fluid particles such as relatively inert and/or catalyst particles in temperature controlled environments contributed in part by the special design of the regeneration system employed in combination with an external catalyst cooler. The cooler design of this invention is used in conjunction with one or more stages of catalyst regeneration as hereinafter more particularly described.